1. Field of the Invention
The present invention relates to a semiconductor light emitting device and its manufacturing method. The light emitting device is used as a semiconductor laser device, as a signal reading and writing light emitting device for use in a compact disk (CD) player and a laser disk (LD) player, as a light emitting device for use in a bar code reader, and as a blue light emitting device (LED) for use in displays of other various electronic apparatuses.
2. Description of the Prior Art
FIG. 1 shows a blue light emitting semiconductor layer device of ZnSe as an example of a conventional semiconductor light emitting device. In the semiconductor laser device shown in this figure, a group II-VI semiconductor of ZnCdSSe or MgZnCdSSe is grown on an N-type GaAs substrate 21. A buffer layer 22 made of N-type ZnSe, a clad layer 23 made of N-type ZnSSe, an active layer 24 made of ZnCdSe, a clad layer 25 made of P-type ZnSSe and an electrode contact layer 26 made of P-type ZnSe are formed in this order in lamination.
On the electrode contact layer 26 which is the top layer, a metal such as Au is directly deposited to form an electrode 27. Similarly, an electrode 28 is formed on the outer surface of the N-type substrate 21. The clad layers 23 and 25 made of N-type and P-type ZnSSe, respectively, function to prevent the diffusion of light caused at the active layer 24.
In a device having such a P-N junction structure of the group II-VI semiconductor, when a bias voltage is applied between the electrodes 27 and 28 in a forward direction, carriers injected by the current are blocked in the active layer 24, so that an induced emission occurs vigorously. When the exciting current exceeds a threshold value, light resonates between the parallel end surfaces of the active layer 24 to cause a laser oscillation. In this case, to improve the output performance of the device, it is important to increase the ratio of the current which contributes to the light emission.
That is, since the light emitting area of the active layer 24 is limited to a central stripe light emitting area 24a, to increase the ratio of the current which contributes to the light emission, it is desirable to limit the spread of the current in a horizontal direction (direction parallel to the active layer 24) to concentrate as much current as possible in the central light emitting area 24a of the active layer 24.
In a conventional device, the current is concentrated in the following manner: In forming the electrode 27 on the P-type buffer layer 26 which is the electrode contact layer, after a metal is deposited, only a stripe area located opposite the central light emitting area 24a of the active layer 24 is left as the electrode 27 and the other portions are removed by etching, thereby restricting the horizontal spread of the current flowing from the electrode 27 to the active layer 24.
In this structure, however, since there is a considerable distance between the electrode 27 and the active layer 24 because of the presence of the clad layer 25 and the electrode contact layer 26 therebetween, the current spreads as shown by the arrows L' before it reaches the active layer 24. As a result, the current also reaches the outside of the central light emitting area 24a of the active layer 24.
For this reason, the ratio of the current which flows through the central light emitting area 24a of the active layer 24 to contribute to the light emission is largely reduced. Consequently, to obtain a necessary current density, a high voltage has to be applied between the electrodes. This not only increases the power consumption but also increases the generation of heat. As a result, the temperature characteristic receives a bad influence.
To restrain the spread of the current to solve these problems, the distance between the electrode 27 and the active layer 24 may be decreased by reducing the thickness of the P-type clad layer 25 and the P-type buffer layer (electrode contact layer) 26 as much as possible. However, in actuality, since the thicknesses of the P-type layers 25 and 26 have to be at least 1 .mu.m to decrease the influence by the absorption and reflection of light caused at the active layer 24, it is difficult to decrease the distance between the electrode 27 and the active layer 24 under present circumstances.
The clad layer 24 blocks the light travelling in a vertical direction from the electrode 27 toward the active layer 24. However, in the horizontal direction, there is no difference in refractive index and light absorption since there is only one layer made of only one component, i.e. P-type ZnSSe. Therefore, there is substantially no light guiding path in the horizontal direction, and the guiding of the light in the horizontal direction is made only by concentrating the current. For this reason, to control the angle of spread of light, there is no other way than regulating the distribution of spread of the current. In such a way, however, it is difficult to minutely control the angle of spread of light.